The death of Robbie the police robot, complete with Gesicht visiting his wife. Mind you it's only chapter 1.
The main character's "child" getting cruelly and pointlessly killed by some fanatic
The death of Mont Blanc is not only saddening on its own, it's also pretty gruesome and gory; mind also that it's chapter 1 as well and we're talking about a robot as well. The homages of the Swiss people are also tremendously heart wrenching.
A flashback, juxtaposing the two families of two robotic children killed by the same guy who also killed Gesicht's child. The human family showed grief and pain emotionally and visibly. The robotic family...they had no visible emotions, however...it's like the Simpsons' quote, "Why was I programmed to feel pain?", except played heartbreakingly straight for drama.
Chapter 6 is like this too. "Don't sing out there. Come back to me. It's time to practice your piano."
Atom's first appearance at the end of the first volume. He just looks so... child-like.
Chapter 24. The robot dog. The little robot dog trying to stand up and play as it's DYING. Hackeneyed or not, DAMN YOU URASAWA.
The efforts of Prof. Ochinomizu to try to repair it are also really sad. Ochinomizu fails to repair it and it dies, prompting the professor to shed tears for its death.
In Chapters 33 and 34, Hercules recalls seeing a shell-shocked robot after a battle, desperately trying to wash his hands under a broken water pipe, saying "It just won't come out."
Chapter 46, Gesicht is utterly heart-broken when he sees Ali who then points a cluster cannon at him.
Chapter 47, especially the ending. "Excellent. It can be for pretend, first... The pretense will turn real with time. You will learn to truly cry, as I do.".
The last few pages of chapter 54. Epsilon throwing his arms away to protect Wassily now defenceless against Pluto. Then wondering where "myself" is killed, Then the guard robot saluting such a brave robot. And then stuff goes From Bad to Worse.
"We're keeping you with us 'til the end of the world."
Brando's death. Even worse is that he's patched in with Atom, Gesicht and Hercules at the time, so they all get to listen to him die. What even makes this worse is the fact that Brando's attempt to give the group anything as to see what they're all dealing with is rendered pointless as Brando's life flashes before his eyes, mainly images of his family who he won't be returning to.
Dr. Hoffman and Helena mourning Gesicht's death. Specially sad, since it didn't have as much in-universe coverage as the deaths of Brando, Hercules and Epsilon. Helena's crying is specially painful as it's the first time she's cried EVER, since she is a robot like her late husband.
Pluto's final sacrifice to destroy Bora and his farewell to Uran... sniff.
Atom's anguished fury: (paraphrased) "Brando didn't want to die!" "Epsilon didn't want to die!" "GESICHT DIDN'T WANT TO DIE!"
And for fans of the source material it's even worse because of there's just so much inevitability about the whole thing. You KNOW what's going to happen and there's nothing that you can do. That's how the story always went and that's how the story will always go. It pulls no punches. Epsilon dies, Gesicht dies.